This invention relates to liquid silicone rubber compositions. More particularly, this invention relates to liquid silicone rubber compositions that prior to cure exhibit good fluidity and excellent moldability and that cure to yield highly flame retardant silicone rubbers that exhibit excellent aging resistance and excellent high-voltage electrical properties such as tracking resistance, arc resistance, and erosion resistance.
Inorganic fillers such as silica micropowder, aluminum oxide powder, aluminum hydroxide powder, and quartz powder are blended into silicone rubber compositions used for applications that demand excellent high flame retardancy and electrical insulating properties. These applications are exemplified by anode caps, plug boots, insulators, and flame-retardant electrical wire and cable. Unfortunately, while high loading levels with inorganic filler can enhance the electrical insulating properties, the resulting silicone rubber compositions have very high viscosities and cannot be used in applications where fluidity is required, for example, as in injection molding.
There have also been attempts to improve the flame retardancy of addition reaction-curing liquid silicone rubber compositions based on low-viscosity polyorganosiloxane, inorganic filler, and a platinum catalyst through the addition of various types of flame retardants. This technology, however, requires a major increase in the addition of inorganic filler in the composition in order to obtain satisfactory flame retardancy and mechanical strength. Moreover, the flame-retardant liquid silicone rubber compositions afforded by this technology have such high viscosities that they can no longer be labeled liquid silicone rubber compositions. Specifically, they cannot be used in applications that demand fluidity, such as injection molding. In order to solve these problems, the preparation of enhanced flame-retardant silicone rubber compositions has been pursued by reducing the addition of highly reinforcing, but also highly thickening inorganic filler, such as silica, and replacing this with a selection from low-thickening inorganic fillers, such as inorganic fillers with large particle sizes. However, liquid silicone rubber compositions prepared using this tactic provide heat-cured silicone rubbers that have reduced mechanical strength and as a result have limited application.
The need therefore exists for a liquid silicone rubber composition that prior to cure evidences suitable fluidity and excellent moldability and that cures into highly flame retardant silicone rubber moldings that have high mechanical strength and excellent high-voltage electrical insulating properties.
As a result of extensive investigations directed to solving the problems described above, the inventors have discovered that the above-described problems can be solved by the admixture, in specific proportions, of specific inorganic fillers and specific flame retardants into a particular type of addition reaction-curing liquid silicone rubber composition.
Therefore, the object of the present invention is to provide liquid silicone rubber compositions that prior to cure exhibit suitable fluidity and excellent moldability and that cure to give a highly flame retardant silicone rubber with excellent high-voltage electrical insulating properties and excellent mechanical strength.